The present invention relates to a liquid-development electrophotographic apparatus which uses a nonvolatile, high-viscosity, high-concentration liquid toner.
As an electrophotographic apparatus operating by the steps of generating an electrostatic latent image on a photosensitive body (a photosensitive drum), causing toner to be attracted to the electrostatic latent image, transferring the toner onto paper or the like, and fixing the transferred toner, a dry-type apparatus, which uses a powder toner, is widely used.
However, a powder toner involves the following problems: toner particles scatter; and since toner particles have a relatively large particle size of 7 xcexcm to 10 xcexcm, resolution is low.
Thus, when high resolution is required, a liquid-development-type apparatus, which uses a liquid toner, is used for the following reason. A liquid toner has a small toner particle size of about 1 xcexcm and exhibits a large electrostatic-charge capacity. Thus, a toner image is unlikely to be disturbed, and high resolution can be achieved.
FIG. 13 shows the overall configuration of a conventional liquid-development-type electrophotographic apparatus (disclosed in, for example, Japanese Patent Application Laid-Open (kokai) No. 2000-56575). In FIG. 13, a photosensitive drum 10 is electrostatically charged by means of a charger 21. Subsequently, the photosensitive drum 10 is exposed to light by means of an exposure unit 22, whereby an electrostatic latent image is formed. A prewetting unit 23 applies silicone oil to the surface of the photosensitive drum 10. Reference numeral 26 denotes a blade for scraping off residual development toner, and reference numeral 27 denotes a destaticizer.
Developing units 24 corresponding to yellow, magenta, cyan, and black are provided and use as a liquid developer a nonvolatile, high-viscosity, high-concentration liquid toner. A developing roller supplies the liquid developer onto the photosensitive drum 10 while causing toner particles contained in the liquid developer to adhere to the photosensitive drum 10 according to an electric field established between the same and the photosensitive drum 10.
An intermediate transfer roller 15 transfers color toners one by one from the photosensitive drum 10 according to an electric field established between the same and the photosensitive drum 10. A heating unit 28 heats the surface of the intermediate transfer roller 15 to thereby melt the toners adhering to the intermediate transfer roller 15. Heating by the heating unit 28 is performed after all color toners have been transferred onto the intermediate transfer roller 15. A pressure roller 19 is adapted to fix on a printing medium the toners which are melted on the intermediate transfer roller 15 by means of the heating unit 28.
When the intermediate transfer roller 15 transfers toner particles from the photosensitive drum 10 according to an electric field established between the same and the photosensitive drum 10, there must be removed oil which is composed of excessive prewetting liquid and carrier in a developed toner layer and which, together with toner particles, is transferred from the photosensitive drum 10 to the intermediate transfer roller 15. For effecting the removal, the illustrated apparatus employs an oil-removing roller 25 on the intermediate transfer roller 15.
A carrier solvent to be used in the liquid-development electrophotographic apparatus is intended to prevent scattering of toner particles, which assume a particle size of about 1 xcexcm, as well as to uniformly disperse toner particles through electrification of the toner particles. In development and electrostatic transfer processes, the carrier solvent serves as a xe2x80x9cbridgexe2x80x9d to facilitate movement of toner particles, which is effected by means of electric-field action.
In a liquid-development printer process, the carrier solvent is a component necessary for storage of toner, transport of toner, formation of a toner layer, development, and electrostatic transfer of toner. However, during and after the step of fixation of toner on a paper medium, the carrier solvent is a component unnecessary for obtainment of good picture quality. Thus, at present, in many liquid developers (liquid toners) a volatile insulating liquid is used as a carrier solvent. However, a liquid developer which uses a nonvolatile carrier solvent is developed in consideration of fixation of toner within apparatus due to volatilization of a carrier, and effects of a volatile carrier on the human body and the environment. An example of such a liquid developer is an HVS (High-Viscosity Silicone) toner.
In some cases, a liquid-development toner which uses a nonvolatile carrier solvent may involve the following problem: the carrier solvent cannot be volatilized during melting of toner through application of heat to the liquid toner and, particularly during fixation of toner or melt transfer of a toner image, hinders development of adhesion of a molten toner onto a paper medium, resulting in a failure to attain satisfactory picture quality and fixation strength with respect to a toner image transferred onto the paper medium.
In some cases, a system that employs melt transfer of an image formed through superposition of toners from an intermediate transfer body to a paper medium may involve the following problem: a toner image on the intermediate transfer body may become spottedly frizzy during application of heat for melting the toners. This problem is related to the relation among releasability of the surface of the intermediate transfer body, viscosity (fluidity) of molten toners, and wettability of a carrier solvent.
Thus, a nonvolatile carrier solvent must be removed to the greatest possible extent before a step of fixing toner on a paper medium is started. However, before a heating step, a limit is imposed on removal of a carrier solvent filling gaps present among toner particles.
Therefore, a xe2x80x9chot carrier removalxe2x80x9d process is effective for removal of a carrier solvent. According to the process, residual carrier trapped in gaps present among toner particles is caused to float on toner particles during melting of toner through application of heat, and the floating carrier is removed. Specifically, in a printer apparatus whose printing operation involves remaining of a nonvolatile carrier solvent within a toner image formed on an intermediate transfer body, the carrier solvent (a liquid component) is separated through utilization of integration of toner particles (a resin component) during melting of toner particles by application of heat as well as through utilization of strong electric-field-induced force of toner particles electrically activated by temperature rise. In application of an electric field to a molten toner in the course of heating, influence of heat on roller members and mutual influence for other process conditions must be considered. However, prior art techniques fail to sufficiently consider these factors in determination of control conditions.
Since color toners are transferred on a single-color basis from the photosensitive drum 10 to the intermediate transfer roller 15, a color toner which has already been transferred onto the intermediate transfer roller 15 passes, before heating, a contact portion between the photosensitive drum 10 and the intermediate transfer roller 15. If the color toner contains excessive prewetting liquid and carrier, the excessive prewetting liquid and carrier are collected and flow at the contact portion between the photosensitive drum 10 and the intermediate transfer roller 15, causing disturbance of an image and affecting heating and melting of a toner layer in the course of fixation. As mentioned previously, the illustrated configuration can remove excessive prewetting liquid or carrier liquid.
However, the illustrated configuration must be such that sufficient cooling is achieved at the contact portion between the photosensitive drum 10 and the intermediate transfer roller 15. From the viewpoint of heat resistance of the photosensitive drum 10, cooling to a temperature of not higher than 60xc2x0 C. must be achieved at the contact portion. In a liquid-development-type electrophotographic apparatus, in order to transfer toner onto a printing medium and fix the transferred toner on the printing medium, toner on an intermediate transfer body must be melted through application of heat from the heating unit 28; however, before transfer of toner onto intermediate transfer roller 15, the intermediate transfer roller 15 must be cooled, since a molten toner causes defective transfer.
Thus, as shown in FIG. 6, a cooling unit, such as a fan, is used for cooling, and in order to facilitate cooling after melt transfer, image formation is performed on a thin belt, whose thermal capacity is small. However, from the viewpoint of maintenance of strength, the belt cannot be made thinner than about 50 xcexcm, which is not sufficiently thin for minimization of thermal capacity; i.e., cooling consumes a large amount of energy.
A melt transfer process for fixing toner on a printing medium is desirably such that, when toner particles are transferred onto the medium through contact with the same, the toner particles and the medium assume a temperature equal to or higher than the melting temperature of the toner particles. In the course of the transfer, a backup pressure is applied to the medium from behind so as to bring a molten toner in close contact with the medium. As a result, the molten toner is transferred onto the medium by means of adhesion of the same.
As mentioned previously, before melt transfer onto a printing medium, excessive carrier oil is removed. In this connection, when an image formation surface is made of a material having a strong toner retention force so as to prevent image impairment which would otherwise result from a removing action effected by a removing roller, an image is not impaired, but in the subsequent step of transfer onto the printing medium the efficiency of transfer is impaired. Conventionally, in printing on a sheet requiring transparency, such as an OHP sheet, fixation temperature is set high, and fixation speed is set low, so as to sufficiently melt toner for increasing fluidity of the molten toner, thereby accelerating integration of the molten toner. However, a drop in printing speed results.
In transfer onto a printing medium, pressure is applied to the printing medium while toner is being heated, to thereby transfer the toner onto the medium by means of adhesion of the toner, and, in order to assist the transfer, an electric field is applied (a bias is applied). However, since the optimum intensity of an electric field depends on a printing medium, an expensive variable-bias power supply must be employed in order to change a bias as needed.
An object of the present invention is to provide a liquid-development-type electrophotographic apparatus in which a toner image on an intermediate transfer body is melted through application of heat and the molten toner image is transferred onto and fixed on a printing medium, the apparatus being characterized in that heat is released efficiently from the same and that accidental spilling of a liquid toner is unlikely to smudge the printing medium.
Another object of the present invention is to provide a liquid-development electrophotographic apparatus adapted to efficiently melt toner particles transferred onto an intermediate transfer body and to simultaneously fix the molten toner on a medium, to thereby provide high efficiency, high fixation strength, and a high-quality image.
Yet another object of the present invention is to enable application of an electric field of the optimum intensity to a printing medium while a bias applied to the printing medium is held constant.
Still another object of the present invention is to perform melt transfer onto a printing medium while thermal influence on a photosensitive body is reduced.
Another object of the present invention is to provide a material, a mechanism, and a condition for effectively and stably removing carrier liquid.
Yet another object of the present invention is to perform optimum carrier removal according to the amount of residual carrier.
Still another object of the present invention is to prevent thermal deterioration of a photosensitive drum through shutting off heat transmission to the photosensitive drum.
A further object of the present invention is to efficiently perform heating and cooling without involvement of a drop in real throughput.
A liquid-development electrophotographic apparatus of the present invention comprises a development section using a nonvolatile, high-viscosity, high-concentration liquid toner as a liquid developer, the development section being in contact with an image bearer body, on which an electrostatic latent image is formed, so as to supply the liquid developer onto the image bearer body, and causing toner particles contained in the liquid developer to adhere to the image bearer body according to an electric field established between the development section and the image bearer body to thereby form a toner image; an intermediate transfer section to which the toner image is transferred from the image bearer body according to an electric field established between the same and the image bearer body; and a transfer-and-fixation section including a heater for melting the toner image transferred onto the intermediate transfer section through application of heat at a contact portion between the intermediate transfer section and a printing medium to thereby melt-transfer the toner image onto the printing medium. According to the present invention, the development section is disposed at a lower portion of the apparatus. Thus, accidental spilling of the liquid toner does not smudge the printing medium and the intermediate transfer section.
According to the present invention, the transfer-and-fixation section is disposed at an upper portion of the apparatus, thereby facilitating release of heat from the apparatus and prevention of thermal propagation into the interior of the apparatus.
A liquid-development electrophotographic apparatus of the present invention comprises a development section using a nonvolatile, high-viscosity, high-concentration liquid toner as a liquid developer, the development section being in contact with an image bearer body, on which an electrostatic latent image is formed, so as to supply the liquid developer onto the image bearer body, and causing toner particles contained in the liquid developer to adhere to the image bearer body according to an electric field established between the development section and the image bearer body to thereby form a toner image; an intermediate transfer body to which the toner image is transferred from the image bearer body according to an electric field established between the same and the image bearer body; and a transfer-and-fixation section including a heater for melting the toner image transferred onto the intermediate transfer body through application of heat at a contact portion between the intermediate transfer body and a printing medium to thereby melt-transfer the toner image onto the printing medium. The intermediate transfer body is equipped with a carrier-removing roller which comes into contact with a toner layer forming an image thereon in order to remove excessive oil from the toner layer and to which a bias voltage is applied in such a direction as to press toner particles against the intermediate transfer body retaining an image. A material having low surface energy (e.g., dimethyl silicone rubber) is used as a surface material, serving as an image formation surface, of the intermediate transfer body, and the electric resistance of the surface material is set to a semiconductive range of 1E4-1E12 xcexa9.
A liquid-development electrophotographic apparatus of the present invention uses a material having low surface energy (e.g., dimethyl silicone rubber) as a surface material, serving as an image formation surface, of an intermediate transfer body, and the electric resistance of the surface material is set to a semiconductive range of 1E4-1E12 xcexa9. In printing on a sheet requiring transparency, such as an OHP sheet, melt transfer is performed without prior removal of excessive oil, and after melt transfer is performed, excessive oil is removed from the sheet.
A liquid-development electrophotographic apparatus of the present invention comprises a development section using a liquid toner as a liquid developer, the development section being in contact with an image bearer body, on which an electrostatic latent image is formed, so as to supply the liquid developer onto the image bearer body, and causing toner particles contained in the liquid developer to adhere to the image bearer body according to an electric field established between the development section and the image bearer body to thereby form a toner image; an intermediate transfer body to which the toner image is transferred from the image bearer body; and a transfer-and-fixation section for melting the toner image transferred onto the intermediate transfer body through application of heat at a contact portion between the intermediate transfer body and a printing medium to thereby melt-transfer the toner image onto the printing medium. The intermediate transfer body is equipped with an excessive-carrier-removing mechanism having a carrier-removing roller for removing excessive oil from a toner layer that forms an image on the intermediate transfer body. The carrier-removing roller comes into contact with the toner layer heated to not lower than a melting temperature thereof or a temperature near the melting temperature, and a bias voltage is applied to the carrier-removing roller in such a direction as to press toner particles against the intermediate transfer body which retains an image.
A liquid-development electrophotographic apparatus of the present invention may be configured such that the intermediate transfer body comprises an intermediate transfer roller for superposing toner images in a plurality of colors through transfer of the toner images from corresponding image bearer bodies, and an intermediate transfer belt having the superposed toner images transferred thereto at one time from the intermediate transfer roller and melting the superposed toner images through application of heat at a contact portion between the same and a printing medium to thereby melt-transfer the superposed toner images onto the printing medium. Rotation of the intermediate transfer belt is controlled according to the amount of residual carrier on the intermediate transfer belt which retains a heated toner layer.
A liquid-development electrophotographic apparatus of the present invention comprises a development section using a liquid toner as a liquid developer, the development section being in contact with an image bearer body, on which an electrostatic latent image is formed, so as to supply the liquid developer onto the image bearer body, and causing toner particles contained in the liquid developer to adhere to the image bearer body according to an electric field established between the development section and the image bearer body to thereby form a toner image; an intermediate transfer section to which the toner image is transferred from the image bearer body; and a transfer-and-fixation section for melting the toner image transferred onto the intermediate transfer section through application of heat at a contact portion between the intermediate transfer section and a printing medium to thereby melt-transfer the toner image onto the printing medium. The intermediate transfer section comprises an intermediate transfer roller to which the toner image is transferred from the image bearer body according to an electric field established between the same and the image bearer body, and an intermediate transfer belt to which the toner image is transferred from the intermediate transfer roller, the toner image transferred onto the intermediate transfer belt being melt-transferred onto the printing medium.
A liquid-development electrophotographic apparatus of the present invention is configured such that the intermediate transfer section comprises a first intermediate transfer body to which the toner image is transferred from the image bearer body according to an electric field established between the same and the image bearer body, and a second intermediate transfer body to which the toner image is transferred from the first intermediate transfer body, the toner image transferred onto the second intermediate transfer body being melt-transferred onto the printing medium.